With the sophistication of optical communication systems, demand has been increased for highly functional optical modules (optical components). A waveguide type optical device can implement various lightwave circuits by forming an optical waveguide on a substrate, and is being used as a component composing an optical module. For the higher functionalization of optical modules, hybrid optical modules are being implemented by integrating waveguide type optical devices having different functions or by integrating a spatial optical component, such as a lens and a spatial phase modulator, and a waveguide type optical device. Specific examples of optical modules include: (1) a V-AWG module obtained by forming an array waveguide grid (AWG) and a variable optical attenuator (VOA) on different planar lightwave circuits (PLCs) and then optically coupling them; (2) a RZ-DQPSK (Return to Zero Differential Quadrature Phase Shift Keying) module obtained by optically coupling waveguide type optical devices made of different materials such as silica-based glass and lithium niobate (LN); (3) a TODC (Tunable Optical Dispersion Compensator) module obtained by optically coupling a LCOS (Liquid Crystal On Silicon) as a spatial phase modulator and a PLC; and the like.
To reduce the impact attributed to external force such as mechanical vibration and shock, a hybrid optical module is fabricated by fixing a waveguide type optical device such as a PLC onto a mount, but has a drawback of thermal stress to be generated by a difference in thermal expansion coefficients between the waveguide type optical device and the mount. PTL1 discloses a technique of reducing the thermal stress against PLC chip connected portions in an optical module that a device to which multiple PLC chips are connected is fixed onto a mount. When explained with reference to FIGS. 1A to 1C (corresponding to FIG. 1 of PTL1), the hybrid optical module of PTL1 has a structure that a PLC chip 2 and a PLC chip 3 are butt jointed while the PLC chip 2 is directly fixed to a convex portion of amount 1. The PLC chip 3 is floated over the mount 1, and thereby, the thermal stress attributed to a difference in thermal expansion between the PLC chip 3 and the mount 1 is suppressed and a coupling loss is reduced at a PLC chip connected portion. A filler 14 is filled between the PLC chip 3 and the mount 1, and the PLC chip 3 is further held by elastic adhesives 9a and 9b applied to the lateral surfaces of holding convex portions 10 mounted on the mount 1. This configuration avoids displacement of the PLC chip 3 in the up-and-down direction even when mechanical vibration or shock is applied in a direction perpendicular to a substrate, which is a direction most affecting the coupling loss at the PLC chip connected portion.